The most important characteristics of optical metrology using the interferometer technique are the non-contact and high resolution. Digital holography is one kind of optical measurement. It replaces the photochemical processing of emulsions with digital processing of the photoelectric signals from a CCD (Charge Coupled Device) array. With the emergence of high resolution CCDs and the improvement of calculation capacity of computers, holography is becoming a well-proven method capable of quickly catching the 3D information of a sample by one-shot measurement with high resolution. Moreover, digital holography is able to record the amplitude and phase of the wavefront from a target object directly to a single image acquired by a CCD camera in a much more time-efficient manner than the scanning electron microscope (SEM) or white light interferometry (WLI) and thus is well suited to the measurement tasks perform on production lines.
There are two types of digital holography, which are on-axis digital holography and off-axis digital holography. In both of the two aforesaid digital holography, a CCD camera is used for recording optically generated holograms that are then reconstructed numerically by a computer program, by that the reconstructed three dimensional holographic image is composed of a real image, a virtual image and a so-called DC term. In the on-axis digital holography, to remove the virtual image and the DC term from the recorded holograms is enabled by the use of a phase shifting method before the holograms are used for the reconstruction. However, it would require at least three holograms for the phase shifting method so that any measurement using the on-axis holography usually will take a longer time. In the off-axis holography, the virtual image and the DC term are separated from the real image by including an angle between the reference beam and the object beam that is greater than zero so that only a signal hologram will be sufficient for the reconstruction. However, the conventional off-axis holography is shorted in that: the correctness of the parameters relating to the angle included between the reference beam and the object beam will have greatly influence upon the reconstruction result while such included angle is limited by the pixel size of the CCD camera.
Moreover, as the conventional digital holography usually adopts a monochromatic light source for illumination, the numerical reconstruction for obtaining hologram will be restricted by the height of the sample. That is, if the height of the sample is larger than half the wavelength of the light emitted from the monochromatic light source, an adverse condition referred as phase wrapping will occur. In addition, when a monochromatic CCD is adopted for catching images, only one hologram can be obtained for each shot so that it will take three shots to obtain three holograms using light sources of different wavelengths, e.g. a red light, a green light and a blue light, so as to be used in an computation algorithm for eliminating the aforesaid phase wrapping. It is noted that the process of eliminating phase wrapping not only is complex and thus difficult to implement, but also can be very time-consuming that seriously affect the measurement speed.
There are already many studies and patents relating to the improvement over the aforesaid shortcomings. One of which is an essay by Myung K. Kim and S. De Nicola et al. at 2003. In this essay, a light source capable of emitting at least two beams of different wavelengths is used as the light source for an off-axis digital holography or a digital holographic microscopy so that at least two holograms can be obtained and used in an numerical computation for reducing phase wrapping and thus the range of surface depth variation of a target object can be detected and measured without being affected by phase noises.
Another such study is an essay by Lingfeng Yu and M. K. Kim et al. at 2004, in which a three-colored LED is used as the light source for an on-axis digital holographic microscopy so that the on-axis digital holographic microscopy is able to reconstruct the profile of a target object by a phase shifting method and thus can be applied for obtaining microscopic three-dimensional images of biological cells.
Yet, there is another essay provided by Myung K. Kim and N. Warnasooriya et al. at 2007. In this essay, a multi-band LED is used as the light source for a holographic measurement by which an axial depth of 7.84 microns can be reached after two computations of synthetic wavelength without causing phase wrapping.
Moreover, another such study is disclosed in U.S. Pat. No. 6,809,845 , entitled “Phase imaging using multi-wavelength digital holography”. In an exemplary embodiment of the aforesaid patent, a two-wavelength laser holography is revealed, by which a axial range of measurement is increased while the effect of phase noise generated from numerical computations is reduced.
From the above description, it is noted that the application of any conventional multi-color digital holographic system in measurement is originated from the need for solving phase wrapping problem, and since most conventional multi-color digital holographic system adopts monochromatic CCD for catching images, only one hologram of one specific wavelength can be obtained by a single shot so that, as it will require to take multiple shots for obtaining holograms of different wavelengths, the processing of the multi-color digital holographic system not only is complex and difficult to implement, but also is very time-consuming that seriously affect the measurement speed.